In a cathode ray tube, the deflection device, also called a deflector, has as its purpose deflecting the electron beam coming from the electron gun so as to make it scan the whole surface of the tube screen and to generate the desired pictures on it.
It is known that in the case where the screen surface is not spherical, or where the radius of a spherical screen is substantially greater than the distance from the center of deflection to the screen, a uniform deflection field generates picture geometry defects which are more substantial as the surface of said screen is flatter. To correct, wholly or in part, these geometry defects, it is likewise known to change the frame and line deflection field of the deflector in order to make them astigmatic.
However, astigmatic deflection fields have the effect of acting on the focussing of the electron beam in such a way that the point of impact on the screen, also called spot, is not circular but undergoes distortions (lengthening along one direction, luminous halo around the central spot, etc.).
These distortions are particularly disturbing to the image definition and need to be corrected for all applications where a high resolution is required.
The astigmatism correction devices of the state of the art comprise pairs of coils which can be wound around rings of ferromagnetic material and disposed at the back of the deflector, partially or wholly around the electron gun.
These devices experience a new limitation due to the higher frequencies of scanning applied to the deflectors, which frequencies may be greater than 64 kHz. Actually, if the astigmatism correction device is disposed at the back of the deflector, around metal parts of the electron gun, these parts act, at high frequency, like an obstacle relative to the magnetic field generated by the correction device. This has the effect of introducing a delay in the establishment of the magnetic correction field, manifesting itself on the screen as a phase error rendering said correction insufficient.
A solution to the problem resides in lengthening the neck of the tube so that the astigmatism correction device is not above the electron gun. This solution is not viable because the market requires a display whose depth is reduced.